Some object fabrication techniques, such as rapid prototyping, build a three dimensional object one layer at a time. A layer of material is formed in the shape of a planar section through the object. Once the layer is formed another is formed in contact with the previous layer. Repetition of this process progressively fabricates an object.
Some processes form objects from a starting material that is a wax or a plastic that can melt. The starting material is melted and the melted material applied. The applied starting material forms a layer of the object on cooling. The object has the chemical and physical properties of the starting material. The process generally requires that the melted starting material has a relatively low viscosity. Starting materials that have a low viscosity in their melted state, however, are generally weak or brittle in their solid state. Weak or brittle objects have limited utility.
Other processes form objects from a starting material that is a liquid photopolymer. The liquid photopolymer is cured to form a layer of the object by exposure to actinic radiation which may be ultraviolet light. Photopolymers may be mixtures comprising acrylate functional monomers and photoinitiators.
Cured photopolymers may become brittle or warp when exposed to ultraviolet light. They may be hygroscopic and bloat in moist environments. Some cured photopolymers may contain residual toxic chemicals and therefore objects formed from these photopolymers cannot be implanted into the body of an animal or human. The range of material properties attainable using photopolymers is limited. Each layer of photopolymer may contract slightly when cured, resulting in cumulative warping and making achievable tolerances insufficient for the production of precision objects such as dental models, dental restorations, tooling patterns and filigree jewelry.
Some other processes form objects from a starter material that is a powder (such as a polymer or ceramic powder) hardened with a binder. The objects are heterogeneous and generally porous, undesirable properties for some objects. The object's physical properties are generally limited to those of the powder. The binder fills the interstices. The binder generally has less strength than the powder, so the objects generally have less strength than the powder. Powder processes generally have a low resolution of at least 0.2 mm which is unsuitable for making fine objects.